In construction machines such as hydraulic shovels, hydraulic units have been known in which a variable displacement pump is coaxially directly connected to a diesel engine mounted, and a required flow rate is increased and decreased by changing the pump displacement (the discharge amount per rotation) without changing the rotational speed of the engine.
A hydraulic unit, which includes an engine and a variable displacement pump that is driven by the engine, will be described below. It is herein assumed that, in this hydraulic unit, the rated rotational speed of the engine is 2,400 rpm, the variable range of the pump displacement is 0 to 54 cc/rev, the allowable pressure of the pump is 25 MPa, and the constant horsepower mechanism setting of the pump is 20 kW. In this case, as shown in FIG. 7, the discharge amount (the flow rate) of the pump is successively reduced from 130 L/min by reducing the pump displacement from 54 cc/rev while maintaining the rotational speed of the engine at the rated value of 2,400 rpm. Since a change in speed of actuators such as a hydraulic cylinder and a hydraulic motor directly relate to a change in pump flow rate. Thus, the pump flow rate is changed as appropriate according to the operation of the actuators.
For example, Patent Document 1 discloses a control apparatus for a construction machine including an engine, a variable displacement pump that is driven by the engine, and pump output control means for performing control so that the product of a load pressure that is applied to the variable displacement pump and a displacement thereof becomes substantially constant.
Patent Document 1: Japanese Published Patent Application No. H11-293710